Overall a nice story, but the part before the witch appears is a bit long and repetitive even if it serves to set up a payoff. Also the writing style was a bit too meandering and overly baroque for my taste. E.g. "the mermaid's face is first submerged, then her torso and, at long last, her tail" - why at *long* last? I guess it could convey how long it felt to Rosette, but I feel like the rest of the sentence covers that sufficiently.

Making the protagonist talk only in indirect speech was pretty interesting.

though I might have trapped you hence - hence means "from here," it's not an oldtimey synonym for here

your charming captive [the princess] - she's the captor, Rosette is the captive.

meniscus - isn't that the name for a water surface in e.g. a narrow tube because it is crescent moon shaped due to capillary action? I don't think it makes sense to refer to the surface of the ocean with it

Also I didn't know you were a Higurashi fan too, cool!